The present application is based upon Japanese Patent Application No. Hei. 3-315607 filed Sep. 24, 1991, a disclosure of which is incorporated herein by reference.
The present invention relates generally to a zoom lens system that is suitable for use with compact cameras which have a smaller constraint on back focus than single-lens reflex cameras. More specifically, the present invention relates to a zoom lens system that features a high zoom ratio of 2.5 and more.
Various types of zoom lens systems have heretofore been known for use with compact cameras. Zoom lenses consisting of three lens groups or more with a zoom ratio exceeding 2 are categorized as follows:
(i) Four-group zoom lens system comprising four lens groups (positive, negative, positive and negative groups), with a stop diaphragm being provided between the second and third groups, all lens groups being moved independently of each other towards the object (this type includes a system where some of the four lens groups are moved in unison). Examples of such zoom lens systems are disclosed in Japanese Patent Laid-Open Publications No. SHO 63-43115, No. SHO 63-159818 and No. SHO 63-157120.
(ii) Three-group zoom lens system comprising three lens groups (positive, positive and negative groups), with a stop diaphragm being provided in the second group, all lens groups being moved independently of each other towards the object. Examples of this zoom lens system are disclosed in Japanese Patent Laid-Open Publications No. SHO 63-153511 and No. SHO 63-161423.
(iii) Three-group zoom lens system comprising three lens groups (positive, positive and negative groups), with a stop diaphragm being provided between the second and third groups, all lens groups being moved towards the object (see, for example, commonly assigned Japanese Patent Application Laid-Open No. HEI 2-73211).
(iv) Practically four-group zoom lens system that has the same composition as the system (iii) except that the second group is divided into a front and a rear group that are movable independently of each other (see Example 3 in the specification of commonly assigned Japanese Patent Application Laid-Open No. HEI. 2-73211, supra).
The above-described conventional zoom lens systems have their own problems. In the system (i), all of the four lens groups have to be moved independently of each other, so a large number of cams must be used; however, it is mechanistically difficult to accommodate those cams in the small space available for lenses for use with a compact camera.
The systems (i) and (ii) require that a shutter block also serving as a stop diaphragm be disposed either between the second and third lens groups (which are subject to substantial deterioration in performance due to manufacturing errors) or within the second group. Under the circumstances, high precision is required for the position of the shutter block while, at the same time, it is difficult to assure consistent optical performance since the imaging performance will be deteriorated greatly in the presence of slight errors.
Further, all systems (i) to (iv) have one problem in common; that is, the overall compactness of those systems is insufficient for using them with a compact camera and in each case, the overall system is composed of as many as 10 elements and more, with at least 5 elements being used in the second group.
The present invention has been accomplished under these circumstances of the prior art and has as an object providing a zoom lens system that features a high zoom ratio of 2.5 or more, that is short in the overall lens length, that is composed of a smaller number of lens elements and which hence is suitable for use with a compact camera.